Chemical vapor deposition (CVD) is currently regarded as the only feasible method capable of producing carbon nanotube structures (CNT) as well as bulk quantities of single-wall carbon nanotubes (SWCNT), double-wall carbon nanotubes (DWCNT), and multi-wall carbon nanotubes (MWCNT). For many applications it is particularly desirable to produce vertically aligned (VA) CNTs. CVD is generally carried out by bulk flow and decomposition of carbon containing gas molecules over small transition metal particles (nanoparticles).
A disadvantage associated with CVD methods is the lack of control over the outcome of the decomposition reaction, originating from two factors. Firstly, in a CVD reactor the heat flow and the mass flow are coupled with the reaction kinetics. Secondly, the CVD growth environment is accompanied by a large number of intermediates formed by homogeneous gas phase reactions. The growth species in CVD must diffuse through a stagnant diffusion layer and the input concentration is no longer related to the concentration of the growth species in a straightforward fashion. Moreover, the homogeneous gas phase reactions produce undesirable byproducts such as amorphous carbon, carbonaceous particles, fullerenes, soot, and inactive metal catalyst particles encapsulated with carbon.
CVD byproducts must be removed by post growth processing to obtain material that is sufficiently pure to be useful for applications. The purification processes are time consuming and expensive, and they consume more than 90% of the material. Currently, the post growth purification efforts take up more time, and are more expensive than the growth process itself.